Effect of biaxial strain on exciton luminescence of heteroepitaxial ZnSe layers

N-doped and Ga-doped ZnSe layers grown by molecular-beam epitaxy on GaAs substrates exhibited double peaks for both the neutral-acceptor-bound–exciton line ($I_1$) and the neutral-donor-bound–exciton line ($I_2$) in low-temperature photoluminescence measurements. The split energy of the double peaks corresponds to an activation energy derived from the Arrhenius plot of intensity ratio of the double peak. Peak energies of the double peaks are calculated from energy shifts of valence bands split by biaxial strain, produced by lattice mismatch and differences in thermal contraction between ZnSe and GaAs. These indicate that the double peak of both bound-exciton lines was yielded from two kinds of excitons related to a light hole and a heavy hole of the valence bands. Thus it should be pointed out that the $I_x$ line generally reported in n-type ZnSe heteroepitaxial layers corresponds to a light-hole branch of usual $I_2$ line.